The present invention relates to a semiconductor integrated circuit equipped with a successive approximation type A/D converter for converting an input analog signal to a digital signal, and particularly to a technique profitable when a high-accuracy analog reference voltage to be supplied to a local D/A converter contained in a successive approximation type A/D converter is supplied from outside a semiconductor integrated circuit.
As an A/D converter (analog-to-digital converter) for converting an input analog signal to a digital signal, there is now known a successive approximation type A/D converter which is product-versatile at low cost and with high performance.
In the successive approximation type A/D converter, a voltage comparator compares an input analog signal and each successive reference voltage. A controller of the successive approximation type A/D converter changes the corresponding successive reference voltage according to the result of its comparison. The voltage comparator performs a comparison with the input analog signal again. The controller executes a successive approximation that it changes each successive reference voltage. A local D/A converter is used for the changing of each successive reference voltage according to the result of comparison. The successive reference voltages are changed by the local D/A converter in response to control digital signals of plural bits from the result of comparison.
As described in a patent document 1 (Japanese Unexamined Patent Publication No. 2004-260263) as one for the local D/A converter, there has been a proposal in which a capacitance array type DAC is used for a main DAC for conversion on the MSB (Most Significant Bit) side of the local D/A converter whereas a resistance string type DAC is used for a sub DAC for conversion on the LSB (Least Significant Bit) side of the local D/A converter, thereby reducing a chip exclusively-possessed area of a semiconductor integrated circuit.
The capacitance array type DAC used as the main DAC of the local D/A converter comprises a plurality of capacitors and a plurality of changeover switches respectively coupled to one ends of the capacitors. The other ends of the capacitors are connected in common and each voltage for common connection is inputted to a voltage comparator as a successive reference voltage compared with an input analog signal. An analog reference voltage is commonly applied to one input terminals of the respective changeover switches, and a base voltage is commonly applied to the other input terminals of the respective changeover switches. The changeover switches are controlled by control digital signals on the MSB side. Incidentally, one capacitor is added to the capacitors of the capacitance array type DAC, and LSB-side successive reference voltages from the sub DAC to be described below are supplied to one end of the one additional capacitor.
The resistance string type DAC configured as the sub DAC in the local D/A converter comprises a plurality of resistances or resistors connected in series and a plurality of switches whose input terminals are respectively coupled to connecting nodes of the resistors. The switches are controlled by control digital signals on the LSB side. Incidentally, output terminals of the plural switches are connected in common so that the LSB-side successive reference voltages from the sub DAC are generated from the output terminals of the commonly-connected plural switches. One end and the other end of a series connection of the plural resistors of the resistance string type DAC are respectively supplied with their corresponding analog reference voltage and base voltage in a manner similar to the capacitance array type DAC configured as the main DAC.